The present invention relates to underwater hydrocarbon production systems.
In producing hydrocarbon products from a reservoir located beneath a seabed, it is usual, once a well has been drilled, to insert production tubing theredown to the reservoir and locate a xe2x80x9cChristmas Treexe2x80x9d containing a number of flow control valves at the mouth of the borehole on the seabed.
It is known for each Christmas Tree, of which there will be one per well, to comprise a number of gate valves, each valve having an associated hydraulic actuator. The Christmas Tree will usually have also one or more temperature and/or pressure transducers. It is common also to interpose a downhole safety valve along with its associated hydraulic actuator a distance beneath the Christmas Tree in the production tubing. A Christmas Tree is expected to operate with minimum maintenance for, typically, twenty years.
In conventional systems, each Christmas Tree would be connected to a topside installation, comprising a hydraulic power unit and a user operated control system, by a composite electrical/hydraulic umbilical, hereinafter referred to as an umbilical. The umbilical contains one or more hydraulic power lines which terminate in one or more electrically controlled hydraulic valves which, on reception of control signals from the topside control system, connect the hydraulic power to the appropriate hydraulic actuators. The umbilical will also comprise an additional line or lines such as electrical conductors for transmittal of control signals from the topside control system and of measurement data from subsea measurement transducers, valve status information and the like, to the control system, and may also comprise liquid flow lines for carrying chemicals from the surface installation for injection into a production flowline or down the well. As the umbilical can be tens of kilometres long, it can be very costly to manufacture and its high weight can make it difficult and/or expensive to install. Although such hydraulic systems tend to be very reliable, they do have drawbacks. Firstly, in an emergency shut down, it may be necessary to bleed hydraulic fluid back up the hydraulic power lines of the umbilical until the pressure has depleted to the point where subsea venting can occur. In a system where a large number of wells are fed by a small number of hydraulic lines, this bleeding may take a significant period of time. Secondly, the hydraulic power unit required at the surface installation is both expensive and bulky. If the surface installation is a fixed or floating oil production facility, where space can be very expensive, the effective cost of maintaining a hydraulic power unit can be significant.
In addition, hydraulic systems of this kind have drawbacks in that hydraulic actuators are usually vented to sea, and that split or holed umbilical lines can also result in large quantities of hydraulic fluid leaking into the sea.
Current oil exploration necessitates the drilling of subsea wells at progressively increasing water depths, with attendant technical problems. For example, the use of hydraulic systems becomes less attractive as they have to be modified to be able to cope reliably with the increasing static head of hydraulic fluid that exist in the hydraulic power lines of the umbilical. Conventional hydraulic systems are also limited in the maximum temperature at which they will reliably operate, making their use in downhole safety valve actuators in high temperature production wells problematic.
In accordance with the present invention, an underwater hydrocarbon production system comprises:
a high voltage alternating current power transmission cable;
a transformer to convert the high voltage power from the power transmission cable to a lower voltage;
at least one electrically powered underwater device;
a remotely located operator station to provide a control signal to control the at least one electrically powered underwater device; and
an underwater control means to control connection of said lower voltage electrical power to the electrically powered underwater device in response to said control signal.
An underwater hydrocarbon production system in accordance with the present invention may allow more flexible system architecture than conventional systems, which may thus allow different applications to be accommodated without extensive system re-design. Also, the underwater components of the system may be lighter in weight than those of conventional systems, allowing more simple and cheap installation and retrieval of these components. Especially as fewer or no hydraulic components are required in this system, the cost of the overall system may be considerably less than that of a conventional system. Using the present invention, it may not be necessary to provide a hydraulic power unit on a topside installation, which can result in significant cost saving. The present invention may also overcome some or all of the drawbacks associated with hydraulic power transmission and/or control of hydraulic devices. Preferably the operator station is surface located.
One or more of said underwater devices may comprise an electrically powered actuator, in which case it or they may be an electro-hydraulic actuator.
The underwater device may have a low power latch associated therewith and the control means may further comprise latch control means operable to control connection of a latching electricity supply to the low power latch. This may allow the use of fail-safe actuators and the like to remain actuated with only low power consumption. The low voltage electrical power may in this case be disconnected from the underwater device when the associated low power latch is energised. This allows the latching electricity supply to be derived from the low voltage electrical power provided by the transformer, which may allow the umbilical to contain only a small number of electrical conductors. Alternatively, the latching electricity supply may be provided by the operator station on a low power transmission cable distinct from the high voltage transmission cable and from the low voltage electricity supply. This would allow emergency shut-down of a Christmas Tree to be achieved from the operator station without involving the underwater control means.
The operator station may further comprise a high voltage electrical power supply to energise the high voltage power transmission cable. This could allow the underwater components of the system to be fully supported by a single umbilical from the operator station. Alternatively, a high power underwater electricity supply is used to energise the high voltage power transmission cable. With the latter alternative, the presence of an underwater electricity supply intended for other underwater electrical power equipment or an underwater high power transmission cable can be used to render an underwater hydrocarbon production system specific electricity supply unnecessary and/or reduce the number of conductors carried underwater from the operator station over the umbilical.
The control signal may be carried on the high voltage transmission cable, which may reduce the number of electrical conductors required in the umbilical, or the control signal could instead be carried on the low power transmission cable, which may provide more reliable and/or higher data rate communications than the case where the control signal is carried on the high voltage transmission cable. This is because there are less likely to be transient signals present on the low power transmission cable, which signals can negatively affect maximum data rates.
Alternatively, the control signal can be carried on an optical fiber. The existence of an optical fiber in the umbilical can allow high communication data rates both to and from the operator station.
The control means may be operable to control connection of said low voltage electrical power to an electrically powered underwater device by providing an address signal corresponding to an address of the underwater device on a fieldbus connected to at least one other electrically powered underwater device.
The control means may further comprise monitoring means to monitor current flowing to the underwater device. Here, the monitoring means may be able to detect anomalies in the current monitored, thereby allowing condition monitoring of the electrically powered devices to be performed.
The system may further comprise electrical protection means operable, on detection of an electrical fault in a component external to the control means, to disconnect electrically that component from the control means. Here, the electrical protection means may comprise means to provide on an auxiliary output a signal thereof indicative of said detection. Preferably, a plurality of said protection means are associated with the control means and the auxiliary output of each protection means is connected to control a respective switching device in a ladder network arrangement.